CuInS2 thin films for photovoltaic : RF reactive sputter deposition and characterization

Abstract

In the present study, we have used the RF reactive sputtering technique to deposit CuInS2 (CIS) thin films, and characterized the sputtered films in terms of structural, optical, and electrical properties, in order to optimize and establish a reactive sputter process suitable for the production of CIS films for photovoltaic applications. The work comprises primarily two aspects: 1) the one-stage growth and characterization of CIS films deposited on float glass substrates; 2) quasi-epitaxial growth of CIS films on single-crystalline sapphire by RF reactive sputtering. Using a Cu-In alloy target and introducing solely H2S as both working and reactive gas during sputtering, we achieved deposition of CIS films on glass substrates in one step. To optimize the sputtering process, we first investigated systematically the influence of sputter parameters, such as the H2S flow rate, substrate temperature, and sputter power, etc., on the film properties. There are mainly secondary Cu-In phases coexisting in the films when the H2S flow rate is insufficient during sputtering, however, excessively injected H2S causes a minor phase of elemental S in the sputtered films. With a RF power of 200 W and a H2S flow in the range of 20-29 sccm (partial pressure: 1.93-2.93x10-1 Pa), highly (112)-oriented films with good quality and very good adhesion can be sputtered on bare float glass at a substrate temperature of 400¢XC or above. A higher H2S flow rate is required to avoid Cu-In alloy phases in the sputtered films when increasing the substrate temperature or sputter power. Raising the substrate temperature from 400 to 500¢XC leads to an improvement of the structural properties of the films, increasing the grain size from ~95 nm to 145 nm, and subsequently enhances the optical properties, shifting the bandgap from ~1.27 to 1.44 eV. With a pre-sputtered Mo- or ZnO-coating CIS films can be sputtered at a relatively low temperature of 200¢XC. The sputtered CIS films are in general oriented preferentially along CIS (112) and have a rough surface due to segregation of secondary phases of CuxS. The Cu/In ratio in the surface precipitates deviates extremely from that in the CIS matrix. Despite of the CuxS surface segregation, the film surfaces remain In-rich with respect to the bulk. Although pure CuInS2-phase films can be produced in one step by RF reactive sputtering, post-deposition treatments affect significantly the structural, optical and electrical properties of the sputtered films. Heating under vacuum at 500¢XC for 2 hours caused recrystallization of the as-sputtered films and consequently improved the film optical properties. Annealing in a H2S atmosphere at 500¢XC for suitable durations resulted in the elimination of the secondary Cu-In phases coexisting in the films sputtered with an insufficient H2S flow. The film structural as well as the optical properties greatly improved. The direct band gap of the annealed films was about 1.49 eV at room temperature, nearly identical to that of bulk materials. Upon exposure to air, the electrical properties of the as-grown films change tremendously, switching from highly resistive electron dominated (n-type) conduction to highly p-type conduction. KCN etching removed the CuxS surface segregations and recovered the film electrical properties to the initial state. The work on quasi-epitaxial growth of CuInS2 films by RF reactive sputtering can be summarized as follows: Thin CIS films (20-100 nm) were ¡§epitaxially¡¨ grown on single-crystalline (0001)-sapphire substrates by RF reactive sputtering at 200W and 500¢XC with an appropriate H2S flow. The well-defined out-of-plane and in-plane lattice ordering of the epilayers was evidenced by XRD rocking curve and Phi-scan measurements, respectively, which determined the orientation relationship of the heteroepitaxial growth: CIS (112) äsapphire (0001), CIS Ë 10 Í äsapphire (10 0). The thin epilayers have a best rocking curve half width of less than 0.05¢X (180 arcsec). Similar to CIS epilayers grown by MBE on Si (111), two domains, rotated by 180¢X with each other, coexist in the sputtered CIS epilayers. The similarity of the hexagonal feature of the CIS unit cell in the (112) plane and the sapphire unit cell in the (0001) plane, and the well-fitted side-length of the corresponding hexagons enabled the heteroepitaxy and led to the observed orientation relationship between the CIS epilayer and the sapphire substrate. AFM demonstrates a smooth surface of the sputtered epilayers and suggests a three-dimensional island-like growth mechanism. Via pre-sputtering of such a thin CIS epilayer, thick CuInS2 films were subsequently grown quasi-epitaxially on sapphire substrates. Such double-layered films have a typical XRD rocking curve half width of approx 0.1¢X (360 arcsec), indicating a nearly perfect ordering of the film along the growth direction. The in-plane lattice alignment is, however, much more complex compared to the thin epilayers. XRD polefigure analysis revealed a multi-domain feature of the double-layered films in the (112) plane. In addition to a primary twin that rotated by 180¢X with respect to the matrix along [221], other domains with different orientations and much lower volumes coexist in the films.